The present invention relates to liquid phase epitaxial processes, wherein it is desired to grow an LPE film upon only one surface of a supporting crystalline substrate.
Liquid phase epitaxial processes generally involve the high temperature growth of single crystal films upon single crystal substrates. Such processes require that the growth, crystal type, lattice size and thermal expansion characteristics be constrained to match the corresponding characteristics of the substrates. The processes thus produce single crystal structures having uniform and continuous lattice arrangements, and also typically having magnetic, optical and compositional characteristics different in the epitaxially grown portion(s) of the structure.
Attendant with these type processes is the requirement of having to immerse the supporting crystalline substrate into the melt, and which step necessarily results in the growth of an LPE crystalline film upon the upper and lower surfaces of the substrate, unless measures were taken to prevent the growth upon one of the surfaces. If steps were not taken to prevent the undesired growth, the common practice employed by those skilled in the art has been to mechanically lap and polish the undesired LPE film from the undesired surface. Such an additional step, though, oftentimes requires numerous substeps, depending upon the thickness of the undesired growth. The lapping step, in general though, requires a great deal of time and reduces the effective yield of any such LPE process, due to the breakage that occurs during the lapping operation.
In particular, during the lapping operation, the undesired crystalline growth is removed via subjecting the undesired growth to a buffing pad upon which successively smaller grits and finally Syton.TM. are applied, until the undesired growth has been removed. As a consequence of this protracted lapping/polishing operation numerous additional problems often occur, such as breaking, cracking, scratching, wedging and lensing of the supporting crystalline substrate and possibly the LPE crystalline film. Consequently, it is desirable that an improved method be developed which either does away with the problem of the undesired growth or minimizes the amount of time and steps of any lapping/polishing operation.
While numerous prior art examples can be found of processes wherein a lapping/polishing operation is employed, little can be found on the subject of not growing the undesired film at all. Relative to an example of an explanation of the lapping/polishing operation, attention is directed to an article by J. M. Robertson, et al, entitled "Garnet Substrate Preparation By Homoepitaxy" in the Journal of Crystal Growth 18 (1973) pp. 294-296. The problem of flux adhesion and the use of acid etching is also discussed in U.S. Pat. No. 3,079,240 issued to J. P. Remeika.
The most common practice for preventing the undesired growth altogether, however, has been to grow the LPE crystalline films upon the exposed surfaces of two supporting crystalline substrates that are placed back to back prior to being inserted into the melt. The theory being that the two substrates will be held together by a strong atmospheric pressure or by a solid/solid self-welding effect, due to the optically flat mating surfaces of the supporting crystalline substrates. An excellent article on this subject by B. S. Hewitt et al, entitled "Technique For Controlling the Properties of Magnetic Growth Films" can be found in the IEEE Transactions on Magnetics, Vol. Mag-9, No. 3, September, 1973, pp. 366-372. It has been applicants' experience, though, that upon employing a back to back method, some flux always penetrates between the supporting crystalline substrates and some minimal lapping/polishing is always required. Further, a great deal of breakage has been experienced, due to weakened substrates. The weakened condition resulting from microfractures that occur during the cooling of the back to back structure; and the microfractures, in turn, resulting from the differential thermal expansion between the substrates and the intermediate flux.
It is therefore a primary object of the present invention to selectively enable the growth of an LPE crystalline films upon only one surface of a supporting crystalline substrate.
It is a further object to prevent the growth upon the one surface via the employment of a gasket between two supporting crystalline substrates, so that the LPE crystalline films will not grow on the surfaces interior to the gasket.
It is a still further object to improve the testability of the characteristics of an individual layer in a multi-layer structure.
These objects and others will become more apparent, as well as their attainment, upon a reading of the following description.